This invention relates to well spiders which are used to engage and hold drill pipe for preventing the weight of the pipe from exerting a downward force into the well and, more particularly, to an improved bowl and slip configuration which increases the life expectancy of the bowl and portions of the slips which grip the pipe and eliminates unnecessary scarring of the pipe.
Spiders are important in well drilling operations because they hold the drill pipe so that the weight of the pipe does not exert a downward force into the well. Although in some cases an inverted spider has been used for holding the pipe against upward movement, the following discussion focuses on the type of spider which holds pipe against downward movement and the terms "upper," "lower," etc. are used in that context although the invention can be applied to and covers inverted spiders.
An essential feature of a spider is a bowl with an opening extending through it in the shape of a conical or pyramidal frustum with the smaller dimension of the opening normally at the lower end of the bowl. Elements known as slips are designed to move between one position where they engage the surfaces which define the bowl opening and grip the drill pipe and another position where they are disengaged from the pipe. When the slips engage the surfaces which define the bowl opening, the downward force exerted by the drill pipe causes a reactive lateral force through the slips in the direction of the pipe for providing a tighter grip on the pipe.
Prior art bowls have typically been formed with an opening having a conical frustum shape and cooperating slips, usually four of them, are moved into and out of the opening. It has been found, however, that although the slips can be formed with outer surfaces which conform to the shape of the conical opening, when the slips engage the bowl and the drill pipe exerts a downward force on the slips, each slip deforms and tends to flatten between its ends which causes excessive wear on the contacting surfaces.
In addition, in one commercially available spider unit the slips are moved into and out of engagement with the drill pipe by pivoting the slips relative to the bowl opening so that whenever the slips are moved into engagement with the pipe the lower ends of the gripping surfaces first engage the pipe and the entire gripping surfaces do not engage the pipe until the slips are in their lowermost positions in the bowl. This results in unnecessary scarring on the pipe and premature wearing on the lower ends of the gripping surfaces.
In another commercially available device slip sections are suspended from a single linkage arm and after the slips are brought into contact with the pipe they are moved around the pipe which also causes unnecessary wear on the pipe as well as on the gripping surfaces of the slips.
In a similar device each separate slip has its own linkage actuated by a ring connected to an air cylinder. The slips are raised and lowered into the bowl through an arc-shaped path so that the lower ends of the slips engage the pipe before it is engaged by the remaining portions of the gripping surfaces. In addition to the drawbacks discussed above, since there is no coordination among the slips two of them normally engage the pipe first and carry the entire load, causing undue stress in the portions of the bowl which support those slips.
In another commercially available device, slips have dovetail-shaped slots on their back surfaces and slide up and down on dovetailed-shaped posts. The posts are designed to replace the bowl and do not have sufficient contact area in order to properly support the weight of the pipe, resulting in high stresses and a short useful life for the apparatus. Such a device is shown in U.S. Pat. No. 2,071,637.
In another development, as shown in U.S. Pat. No. 2,527,954, a well spider has been formed with a bowl opening in the shape of an inverted pyramidal frustum with four sides. Two slip sections are pivoted together and are raised and lowered relative to the bowl in such a way that the gripping surfaces of the slips do not necessarily engage the pipe uniformly across the surfaces during all phases of the operation. Further, the bowl is formed in two sections which are hinged together and require a relief surface to be machined into the intersecting pyramid surfaces in each section. This additional machining step significantly adds to the difficulty of fabricating the bowl and increases the production costs.